Physisorption and Chemisorptions

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Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption. The fundamental interacting force of physisorption is caused by van der Waals force. Even though the interaction energy is very weak physisorption plays an important role in nature. For instance, the van der Waals attraction between surfaces and foot-hairs of geckos provides the remarkable ability to climb up vertical walls.Van der Waals forces originate from the interactions between induced, permanent or transient electric dipoles.

Chemisorption or chemical adsorption is adsorption in which the forces involved are valence forces of the same kind as those operating in the formation of chemical compounds. The problem of distinguishing between chemisorption and physisorption (see below) is basically the same as that of distinguishing between chemical and physical interaction in general. No absolutely sharp distinction can be made and intermediate cases exist, for example, adsorption involving strong hydrogen bonds or weak charge transfer.

Distinguish between Physical Adsorption and Chemisorption

The forces operating in these are weak vander Waal’s forces.	The forces operating in these cases are similar to those of a chemical bond.
The heat of adsorption are low i.e. about 20 – 40 kJ mol-1	The heat of adsorption are high i.e. about 40 – 400 kJ mol-1
No compound formation takes place in these cases.	Surface compounds are formed.
The process is reversible i.e. desorption of the gas occurs by increasing the temperature or decreasing the pressure.	The process is reversible i.e. desorption of the gas occurs by increasing the temperature or decreasing the pressure.

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